It is well known that zeolite promoted hydrocarbon cracking catalysts are highly active and selective for the production of gasoline from gas oil type feedstocks. The gasoline fractions that are produced by the zeolite catalyst possess moderate clear octane readings and generally require the addition of anti-knock components prior to being used as a motor fuel.
It is thus a primary objective of petroleum refiners to select catalysts and catalytic cracking conditions which maximize the formation of high octane gasoline components during the cracking process. The attempts to prepare these catalysts have not always been successful and sometimes have led to the preparation of catalysts that have undesirable characteristics such as low catalytic activity or high coke and hydrogen production.
U.S. Pat. 3,788,977 of Dolbear describes a process for preparing a catalyst that has an improved aromatic producing capability by impregnating an inorganic oxide base such as alumina with from about 1 to 20% uranium oxide generally in combination with about 0.01 to 1% platinum. The product recovered from the process described in this patent is best considered as an additive that might be used to increase the octane ratings so as to decrease the amount of tetraethyl lead required to enhance the anti-knock performance of the gasoline.
Typical zeolite promoters are described in the prior art and comprise hydrogen or rare earth exchange synthetic faujasites which have a silica to alumina ratio of about 2.5 to 6. These faujasites that are designated type X and type Y by the Linde Division of Union Carbide Corporation are prepared from synthetic faujasite and are generally available in the alkali metal form. To prepare the cracking catalyst additive the alkali metal faujasites are normally exchanged with solutions containing hydrogen and/or rare earth ions, subsequently calcined and finally washed or ammonium exchanged to reduce the soda level of the product to below about 0.5 weight percent. Typical rare earth exchange faujasites and the preparation thereof are disclosed in U.S. Pat. No. 3,402,996 to Maher et al.
U.S. Pat. No. 3,293,192 to Maher et al. discloses another type of treatment of the zeolites in which the zeolites are modified by thermal treatment.
Commercial cracking catalyst compositions which are readily available contain in addition to the zeolite promoter matrix type material such as amorphous silica, alumina and/or silica alumina hydrogels. The matrix component may also contain or comprise clay such as kaolin, or thermally or chemically modified clays such as metakaolin. These catalysts have a wide variety of commercial designation such as XZ-15, XZ-25, XZ-36, XZ-40, DZ-7, DZ-5 and other commercial designations.